The present invention relates to the quantitative metering of materials capable of flowing naturally, such as grained products or pulverulent products. The metering of such products, whether it be volumetric metering, or weight-based metering, has a large number of industrial applications. In some cases it is highly desirable for the metering to be carried out continuously in order to be able to incorporate the metered material in a continuous production process.
Although quantitative metering has been completely mastered for liquid phase products, the same cannot be said for products in powder form. Systems which operate by weighing are already known, such as the system described in the U.S. Pat. No. 4,320,855. Such a system comprises a reservoir containing the material to be metered, the total weight of which is continuously evaluated. A flow of material is extracted continuously from this reservoir, and knowledge of the progression in the lowering of the total weight makes it possible to regulate the output of material withdrawn from this reservoir.
Unfortunately, such a system is not sufficiently accurate. Indeed, the scales used must be capable of measuring the total weight of material contained in the reservoir. Furthermore, it is desirable to be able to measure accurately the instantaneous quantities of material withdrawn from the reservoir. Now, the quantities of materials withdrawn per unit of time represent typically a very small percentage of the total mass of material which the reservoir is capable of containing. It is known that, in metrology, it is not generally possible to achieve an accuracy of the order of one percent on a deviation which is itself only a very small percentage of the measurement range of an apparatus.
It is therefore very difficult to perform a very accurate metering of material on the basis of a method such as that described in the aforementioned U.S. Pat. No. 4,320,855. In addition, it may be noted that a high degree of accuracy will be all the more difficult to achieve in a continuous method where the output is very small: since a continuous output is desired, the flow will be divided into as small as possible unit quantities; in dynamic operation, achieving an accuracy of better than a few percent on a quantity of a few milligrams every second is problematic.
Metering devices are also known which operate according to a principle of volume measurement. Now, most of the time, the metering which must be carried out is in reality a weight-based metering, because clearly defined masses of material are mixed in order to form a product of given composition. Accordingly, volumetric metering is only an indirect approach to the weight-based metering which must in fact be performed. Consequently, it is often necessary to carry out prior packing of the material to be metered, in order to have a strictly constant density of material upstream of the volumetric metering system.